The security of containers has become an important issue in today's security-conscious environment. This is particularly apparent in the shipping of containers across borders. The locking of loaded containers to prevent unauthorised access after the containers have been loaded and before the containers reach their destination is now seen as an important, if not mandatory security precaution.
Of course, a lock on a container may be removed and then replaced. Therefore, it is important that tampering with a lock is able to be detected. To assist with the identification of tampering with locks, electronic seals have been utilised.
One form of electronic seal that has been used in the past is an electronic tagging device that wirelessly transmits information to an interrogator. This information identifies whether the lock has been tampered with.
U.S. Pat. No. 6,265,973 (Brammall et al.) describes an electronic security seal. A conductor along the bolt shank is connected to a circuit and provides a tamper evident signal to the circuit when the bolt is severed. The circuit senses removal of the bolt or severed bolt condition and generates a “tamper” signal, which is transmitted to a local receiver/reader.
U.S. Pat. No. 6,747,558 (Thorne et al.) describes a method and apparatus for providing container security with a tag. A device includes a bolt, which extends through openings in a latch mechanism on the container. The bolt also passes through spaced coils of the seal device. The sealed device uses one coil to generate a magnetic field, while monitoring the corresponding magnetic field induced in the other coil. Tampering with the bolt affects the magnetic field, which in turn permits the seal device to detect the tampering. The seal device periodically transmits wireless signals, which can be remotely received for the purpose of tracking the container and monitoring the integrity of the seal.
A major disadvantage of providing sophisticated electronic security seals is the increased cost involved in shipping containers. Even if the devices are made to be reusable, there is the associated cost, inconvenience and possible additional security issues related to the reuse of electronic seals. In addition, electronic seals that actively transmit signals may be subject to stringent regulations regarding the maximum power of transmission and the frequency bands in which transmissions may be made. These regulations may change from jurisdiction to jurisdiction and over time. There is also the problem of passive transponders interfering with the signals from other passive transponders when interrogated. This problem remains even in applications where evidence of tampering with a lock is not required.
Apart from the sealing function of electronic seals, there are many other applications where removal or tampering with a value item needs to be detected.
It is therefore an object the present invention to overcome or ameliorate problems with electronic seals and/or transponder devices at present, or at least to provide the public with a useful alternative.
Any reference in this specification to the prior art does not constitute, nor should it be considered, an admission that such prior art was widely known or forms part of the common general knowledge in Australia, or in any other jurisdiction, before the priority date of any of the appended claims.